The present invention relates to a net and a method of producing same. In more particular the invention relates to a net derived from a film or sheet.
Nets either derived from film or formed directly at the exit of an extrusion die are at present mainly used as reinforcement in paper articles or cheap textiles, e.g. in sanitary paper articles, paper cloth and heavy duty bags, and non-woven upholstery fabrics. They are further used, or have been suggested for use as window curtains, decorative net, wrapping material, mosquito net, protective gardening nets against insects or birds, backing for growing of grass or plants, sport nets, light fishing nets, and filter materials.
The nets formed directly at the exit of an extrusion die have the drawbacks or limitations that the mesh is relatively big, that orientation of the cross-points is difficult, and that the nets can generally not be made finer than about 20 grams per sq.m.
One type of net derived from film consists of two or more cross-laminated layers of uniaxially oriented film which have each been fibrillated to net form. This involves the need for a lamination process after fibrillation and at this stage lamination is generally complicated. For reinforcement in sanitary paper--a fineness of 5 grams per sq.m. or finer is usually needed, and it is practically impossible to obtain this when two layers have to be laminated after fibrillation.
In a modification of this type of net, lamination can be avoided, as reinforcement filaments of a non-splitting polymer is incorporated in the splitable film, which is oriented at an angle to the incorporated filaments and then fibrillated at random. However, the randomness of the mesh is usually a drawback, and the randomly fibrillated material is relatively weak, as special admixtures are used to facilitate the random fibrillation.
Another type of nets derived from film is formed by slitting a pattern of intermittent lines, which are mutually staggered, and expanding the slit film while stretching biaxially. This process produces nets of a big mesh and with weak cross-points.
A third type of net derived from film is formed by embossment of the film in a suitable pattern followed by biaxial stretching under conditions which fibrillate the thin areas of the film at the same time as the material is expanded. Dependent on the pattern of the embossment different properties can be obtained, but the net is generally weak as the bosses remain unoriented.
A fourth type also makes use of embossment, but the bosses are formed as transverse stems which are subsequently oriented and form the main direction of strength. The thin, lineary areas of the embossed film are oriented and split generally in the longitudinal direction and form relatively weak bridges between the transverse stems. For most uses this net has to be cross-laminated with another net or web, and this again involves the limitation that it is very difficult to produce the very low sq.m. weights required for reinforcement in sanitary articles.
Finally, it is known that net structures can be produced by extruding alternate streams of different polymers, joining the streams in the extrusion die, which contains moving parts that turn the streams to a transverse direction at the same time as they draw the streams to thin structures named "lamellae". Still in the die, one or both boundaries of the fluid sheet pass over a comb which drag longitudinal "tentacles" from the edges of the lamellae and collect the tentacles in bundles. After solidification, the sheet is mechanically disrupted in accordance with the layered and fibrous structure produced in the die, and hereby a fabric is achieved with two interconnected thread directions. However, the fabrics produced by this method are several times heavier than the abovementioned usual requirements for nets in sanitary articles.